The invention relates to a method for setting a multiplication of a continuously variable automatic transmission (CVT) equipped with a variator for a vehicle having an electronic transmission control (EGS) which has a control unit, with a microcomputer connected with sensors and/or external control units to detect several operating parameters, and with an actuator wherein a starting clutch is actuatable by the actuator.
From the practice methods are known for adjusting the multiplication of an automatic transmission in which, on the basis of specific input variables on the electronic transmission control (EGS), such as accelerator pedal position, motor rotational speed, rotational speeds of primary and secondary disks of a variator and the like, the transmission multiplication is set by means of characteristic fields stored in the EGS for a driving strategy under usual operating conditions.
For multi-step automatic transmissions, there are also known from the practice, special programs for particular operating conditions such as a winter driving program manually actuatable by the driver, via a switch, for better starting with low friction values, i.e. a snow-covered or iced road. Hereby is prevented, among other things, shifting in the lowest gear as a starting multiplication.
The known winter driving program, however, cannot be used in automatic transmissions having a variator, since the multiplication range of the transmissions is substantially limited by the mechanical stops of the pushing linked band of the variator with the shortest possible multiplication (LOW) and longest possible multiplication (overdrive, OD). Compared to multi-step automatic transmissions CVT transmissions usually have a larger spread accordingly making shorter starting multiplications possible, which nonetheless are a cause of low starting power and poor starting stability in the presence of low road friction values.
Besides, it has proven problematic in automatic transmissions having a variator that at extreme retardations of the driving speed, such as emergency braking or engagement of an antiblock system (ABS), the setting of the multiplication is so slow that when an acceleration operation immediately follows braking, there is not an optimum multiplication of the transmission.
The starting power when restarting is accordingly weak as a result of a still too low multiplication ratio. In addition, during an extreme retardation of the driving speed, there appear on the drive train high drag torques which impair the driving stability of the vehicle.
Therefore, this invention is based on the problem of making available a method which makes possible a quick adaptation of the multiplication of an automatic transmission with a variator (CVT transmission) to a critical driving situation, mainly in the cases of low road friction values and abrupt decelerations, a strong starting power being ensured, and at the same time the drag torques of the drive train can be reduced whereby, in turn, a high braking stability of the vehicle is ensured.
With the aid of the inventive method, which can be implemented in a regular driving strategy, a free multiplication adjustment is advantageously possible for setting a multiplication adapted to the need when starting a motor vehicle or when restarting after a strong brake operation.
By opening the starting clutch, the variator is uncoupled from the motor or the output and, in this manner at least, an additional degree of freedom is obtained.
The action of the inventive method is, in particular, dependent on the transmission type, i.e. on whether the starting clutch that conveys the power flow for forward drive is situated in the transmission on the secondary side, hence on the output side, or on the primary side and thus on the motor side.
If the starting clutch is on the secondary side of the variator, the latter is uncoupled from the output and the wheels when the starting clutch is opened.
Thereby the drag torques of the part of the drive train connected with the wheel are advantageously reduced when decelerating, and the variator can be adjusted, independent of the output rotational speed, to a multiplication suitable to an actual vehicle speed, with which it is possible to continue driving after having applied the inventive method.
The use of the inventive method in the arrangement of the forcelocking forward shifting and starting clutches on the secondary side proves especially advantageous when the variator is adjusted while the vehicle is parked. The variator, which runs at the motor rotational speed, can, of course, be easily adjusted when the starting clutch is completely or at least partly open during a stopped adjustment of the variator and when the variator is uncoupled from the wheel rotational speed.
The circumstance described precisely predestines the inventive method, e.g. for a manually selectable winter program with a presetting of the multiplication prior to starting with a low road friction value, whereby the starting power and the starting stability are considerably improved.
The inventive method, however, is also adequate for other applications. It is very advantageous in braking operations with a sharp reduction of the vehicle speed, as in an engagement of an antiblock system (ABS).
It is also possible, by the inventive method, to superimpose a regular driving strategy to improve the driving stability of the vehicle, the variator being separated from the output or wheel by the opening of the starting clutch.
Thus the drag torques of the drive train can be effectively reduced and the variator can correspondingly be freely set to the multiplication suited to the actual vehicle speed or the output rotational speed of the transmission, with such multiplication driving is continued after terminating the program.
The advantages of the inventive method are optimally prevalent in combination with an arrangement of the starting clutch on the secondary side.
However, the inventive method also has advantages when the starting clutch is disposed on the primary side of the variator, e.g. in the above described situation of an ABS engagement.
The variator here remains connected with the output or wheel. The adjustment in the direction of the longest possible multiplication (OD) reduces the slowness of the drive train connected with the wheel. By opening the starting clutch, the variator is uncoupled from the motor whereby the drag torques of the drive train are advantageously further reduced.
In addition, via a function within the electronic motor control, the motor can advantageously be guided to a defined rotational speed in order to ensure the oil supply for a quick variator adjustment. This advantage results from a transmission oil pump operated at the motor rotational speed, since a higher motor rotational speed results in a quicker pressure build up.
The inventive method constitutes a universally useful program module for an automatic transmission, it is particularly adequate for a CVT transmission with a belt drive variator, but it obviously can also be used in automatic transmissions with different types of variators.